Deepwater oil and gas exploration and production projects face many unique challenges that impact associated production facilities and drilling activities. Hurricanes and loop currents rank high on the list of factors that hinder deepwater operations. Hurricane and loop currents shorten the operability envelopes for drilling activity, and shutdown operations. They also can cause system failure.
Moored facilities have operability limitations compared to those that are dynamically positioned (DP):                Unable to drift with current or weather-vane to aid running/retrieving drilling riser in presence of underwater currents        Interference with diverter housing prevents running/retrieving drilling riser even in relatively low underwater currents        Unable to move away from a storm        Preparation time for storms is much longer        Running/retrieving operations on a moored drilling vessel may not be possible with currents exceeding about 1.0 knot        Big problem during hurricane season when weather can deteriorate rapidly        Deploying drilling riser in a region where loop currents can occur in combination with hurricanes presents a potential risk to the platform        
Referring to FIG. 1, BP America's ThunderHorse facility is depicted. ThunderHorse a semi-submersible 50 having a derrick 52, and a main or drilling floor or deck 54. Additional details are shown in FIG. 2.
Sitting in 6,000 ft (1,829 m) of water about 150 ml (241 km) offshore, the ThunderHorse production-drilling-quarters (PDQ) semisubmersible is the largest production semi ever built, with a total displacement of 130,000 tons (117,934 metric tons). The topsides area of ThunderHorse is the size of about three football fields, and is packed with equipment and systems to treat and export 250,000 b/d of oil plus associated gas.
Harnessing ThunderHorse posed challenges in almost every aspect of development. Everything is interrelated and, as a result, you can't do anything in isolation. A very well defined and coordinated approach involving every aspect of a task is required. Even small issues can quickly magnify because of the compounding effect.
ThunderHorse is located in ultra deep waters with both loop currents and the threat of hurricanes. The project must also contend with reservoir temperatures up to 270° F. (132° C.), pressures up to 18,000 psi (124 MPa), and a reservoir with flow rates of up to 50,000 b/d of oil/well. As a result, ThunderHorse required larger bore tubing inside the wells than is normally used in the Gulf of Mexico and a very large, long and heavy riser assembly.
There are three basic operating modes for drilling risers:
1. Connected
2. Fully Retrieved
3. Hung-off
In the event of excess underwater currents, operators often want the drilling riser/LMRP (Lower Marine Riser Package) either fully retrieved or connected. If a hurricane is expected, operators usually want drilling riser/LMRP fully retrieved or hung off. Depending on magnitude of storm, in a connected mode, the upper flexible joint might exceed operational limits. If the riser is hung-off in loop currents, fatigue life is dramatically reduced. Nevertheless, it is often preferable to remaining connected during a hurricane.
Stopping all work and retrieving the riser, while conservative and safe, is clearly a high cost option. It also requires a large amount of deck space which leads to a larger and more expensive facility. There is also the risk of equipment damage during the retrieval and the possibility of dropped objects.
Disconnecting the riser and using a parking pile is another possibility. However, riser fairings may be needed and riser tensioners may have to be modified to allow the riser to stroke.
This problem has existed for some time. Considerable effort has been made, and significant amounts of money have been expended, to resolve this problem. In spite of this, the problem still exists. Actually, the problem has become aggravated with the passage of time because more facilities are drilling in deeper and deeper parts of the world and hurricanes have been increasing with greater force and frequency.